• This theory is base on the assumption that seeping water through the soil below the weir
  • Follows the path along with the contact of the base, with the underlying sub-soil.
  • The length of the path of seeping water from the point of entry into the sub-soil from the U/S of the impervious.
  • Apron to the point at the D/S end of the impervious apron is know creep length.
  • Bligh also assumed that loss of head of the seeping water is proportional to the length of its travel irrespective of whether.
  • The length of travel is in the horizontal or vertical direction.
  • He also assumes that unless cut-off sheet piles extent to the impervious subsoil strata, no amount of sheet piling could stop the flow of percolating water.
  • AB is the length of impervious apron l and H is the head of water-fill up to the top of the weir CD and there is no water on the D/S side.
  • According to Bligh’s theory L = l where L is the total creep length.
  • If vertical cut-offs are provide below the impervious apron.

\[L= 1+2d_{1}+2d_{2}\]

  • The Length of vertical cut-off  taken double because vertical cut-off provides the creep length equivalent.
  • To twice the length of the cut-off, as seeping water once goes down and then comes up along the cut-off.
  • If H is the total head causing seepage or total loss of head, and L the total creep length, the loss of head per unit length of creep (C) is given by

\[C= \frac{H}{L}= \frac{H}{1+2d_{1}+2d_{2}}\]

  • Loss of head per unit length of creep (C) is know  percolation coefficient.
  • The reciprocal of percolation coefficient is know the coefficient of creep (C).
  • Safe values of coefficient of creep for different soils are given below.

Coefficient of Creep

Weir design by Bligh’s theory.

  • According to Bligh’s theory, two design criteria are to be consider.

(i) Safety against undermining or piping.

  • To safeguard the weirs against failure by piping, the creep length should be provid according to the following formula:

L = CH, where C is coefficient of creep.

  • Such a length would provide safe hydraulic gradient and seeping water emitting from D/S end of the impervious apron.
  • Will not be having sufficient uplift pressure to dislodge the soil particles.
  • This will avoid boiling action of soil at the D/S end of the apron.

(ii) Safety against uplift pressure.

  • To counter balance the force of uplift, sufficient floor thickness should be provid, specially on the D/S side of the weir.
  • Let at distance L1 creep length from U/S end of the impervious apron h is the resultant uplift head, the net uplift pressure wh can be comput follows